This invention relates to a novel joint construction and in particular to a structural beam construction utilizing a novel joint between the flange and web components of such beam. The beam may be of flanged construction, such as I-beam, T-beam or angles. The beam components may be dimensional lumber, composite wood, synthetic materials or any combination thereof.
It is well known in the art to manufacture composite I-beams from wooden material wherein flange or chord members are made of dimensional lumber and are joined to planar webs. The webs may be formed of dimensional lumber, but typically are formed from elongated, planar panels of plywood, particle board, waferboard, or oriented strand board. Typically in an I-beam, the dimensional lumber flanges are positioned parallel to one another with the grain extending longitudinally of the flanges. Web material is fixed between the flanges. Where plywood material is used for the web, the grain of the outer plies preferably extends parallel to the longitudinal direction of the flanges, although it is known to orient the outer plies perpendicular to the flanges. Similarly, where oriented strand board is utilized, it is optional and preferable to ensure that the grain directions of the outer layers are oriented parallel to the flanges.
Also typically in such wooden beam construction, the joints between the webs and flanges comprise tongue and groove construction, or a modification thereof. U.S. Pat. No. 1,377,891 to Knight illustrates rabbet as well as dovetail joints in a wooden I-beam. Such joints may be glued or mechanically fastened to retain the tongue within the groove. The joint constructions typically have comprised either a clearance fit, or an interference or friction fit. As a result of normal fluctuating moisture contents in wood, dimensional stability of the joint structure is difficult to ensure and tolerances must be generous. Consequently, clearance fit joints frequently are either undersized or oversized. If undersized, joint weakness results from reduced glue attachment, and if oversized, a friction fit develops with the associated problems of such construction.
Although friction fit jointed wooden I-beams have achieved significant commercial acceptance typically as joist or truss members, there are certain manufacturing and application problems associated with them. In friction fit joints, a tongue structure may be formed on the edge of the web, which then may be inserted into a groove structure formed longitudinally within the flange member. A unitary tongue member may be inserted straight into a matching groove, for either a clearance or interference fit. Alternative designs include bifurcated or parallel tongue members which may be forced into cooperating groove means so that the legs of the tongue are biased or flexed inwardly towards each other or outwardly away from each other. In all such cases, a degree of compression is applied laterally to the tongue members.
For example, U.S. Pat. No. 4,191,000 to Henderson discloses a joint with a specially milled bifurcated tongue inserted into a mating bifurcated groove. U.S. Pat. No. 4,456,497 to Eberle discloses a joint where the parallel legs of a tongue are squeezed together in a tapered groove. U.S. Pat. No. 4,195,462 to Keller discloses a joint where paired parallel legs of a tongue are flexed apart upon insertion into paired inclined grooves.
In the usual case where glues are utilized as the adhesion means between the web and the flange, the frictional interfitting of tongues and grooves results in scraping of the joint surfaces and removal of the glue over a greater or lesser extent of the surfaces of the joint. Such removal results in glue starvation and ultimate weakness of the joint connection.
This problem has been made recognized in the past, and attempts have been to overcome the problem by providing specially designed surfaces to reduce glue removal and allow it to flow within the joint area. U.S. Pat. No. 4,336,678 to Peters discloses a scalloped or dimpled surface on the lateral edges on the tongue of the web to vent the glue line and permit more even application of glue. Similarly, U.S. Pat. No. 4,715,162 to Brightwell discloses a tapered tongue on a web member, wherein glue slots are provided.
A further manufacturing problem associated with friction fit joints is the tendency of such joints to disconnect or separate during the assembly and adhesive curing stages of manufacture. Where compressive forces are required to insert the tongue into the groove of the joint, it has been recognized that the glue may act as a lubricant, thereby reducing the frictional adhesion between mating surfaces of the joint and facilitating separation of the joint unless great care is taken to clamp or otherwise retain the I-beam components in position during the curing stage. This additional requirement increases the cost and complexity of manufacture and assembly of wood I-beams. U.S. Pat. No. 4,191,000 to Henderson discloses the use of a nailed assembly to retain the integrity of the joint during the glue curing stage.
Frictional joints also create a lateral or transverse cupping effect in the longitudinal flanges which results in undesirable application and aesthetic problems. The transverse pressures in a wooden flange resulting from the frictional compression at the joint interface will result in spreading of the inner flange surfaces adjacent the web relative to the outer flange surfaces opposite the web. This unequal expansion generates a bow or cup in the flange. The cupping effect may be exacerbated by further expansion of the wood fibres adjacent the joint in consequence of absorption of moisture from the liquid glues typically used in the manufacture of wooden components. Such cupping of the flange may result in twisting and instability of the beam, undistributed loading in application, and basic cosmetic degradation.
Still a further problem associated with flanged beams manufactured from dimensional or sawn lumber is the unevenness of surfaces and dimensional irregularities such as warps, cupping and bows. In contrast, man-made composite materials such as plywood, particle board, waferboard, laminated veneer lumber (LVL) and structural composite lumber have more consistent dimensional parameters. The joint of the present invention is particularly well suited to sawn lumber flanges.